1. Field of the Invention
The present invention relates to a nitride semiconductor laser chip and a method of fabrication thereof; in particular, the present invention relates to a ridge-waveguide type nitride semiconductor laser chip and a method of fabrication thereof.
2. Description of Related Art
As materials for light-emitting chips for use as short-wavelength light-emitting chips such as semiconductor laser chips and light-emitting diode (LED) chips, nitride semiconductor materials such as GaN have been researched and developed. Generally, GaN-based semiconductor laser chips using a nitride semiconductor material have a structure in which an InGaN layer is included in an active layer, and such laser chips are already in practical use as light sources for data reading in optical disc devices.
Nitride semiconductor laser chips for use in optical disc devices and the like usually have a ridge portion for confining light in the horizontal direction, and this ridge portion is formed to have a real index guide structure in which the ridge portion is buried under an insulating film such as a SiO2 film.
Here, it is known that, in semiconductor laser chips, increasing the amount of electric current injected with a view to raising the optical output will cause the semiconductor laser chips to oscillate (lase) not only in the fundamental mode but also in higher-order modes. For this reason, in conventional nitride semiconductor laser chips, to suppress higher-order modes and for other purposes, the ridge portion is designed to have a ridge width as small as about 1.5 μm.
With a view to further suppressing higher-order modes, in other conventionally proposed semiconductor laser chips, a light absorption layer is formed in contact with a nitride semiconductor layer. Such nitride semiconductor laser chips are disclosed in, for example, JP-A-H11-186650, JP-A-2002-270967, JP-A-2005-223148, and JP-A-2008-91910.
In optical disc devices, laser light is shone on a disc, and the reflected light is received by a light-receiving element, and thereby the recorded information is read out. Here, for some reason, the reflected light may return to the semiconductor laser chip. If this returning light enters the active layer, the semiconductor laser chip will become unstable, causing fluctuation in light intensity and other inconveniences, thereby producing noise. For this reason, in cases where semiconductor laser chips are used in optical disc applications, they are driven by use of a high-frequency superimposition circuit as a measure against noise.
Inconveniently, however, with the conventional nitride semiconductor laser chips mentioned above, since they have high device resistances, unless high-frequency superimposition is applied amply, optical disc devices do not operate properly. Thus, a high-frequency superimposition circuit needs to be one that can drive a semiconductor laser chip at high frequency and large amplitude, and is therefore expensive. This, inconveniently, makes cost reduction difficult. Moreover, the high device resistances of the conventional nitride semiconductor laser chips require high operating voltages, and hence, inconveniently, lead to high electric power consumption.
On the other hand, in some conventionally proposed nitride semiconductor laser chips, to reduce the operating voltage, an electrode is formed so as to cover the top surface and side walls of the ridge portion. Such nitride semiconductor laser chips are disclosed in, for example, JP-A-2010-34246. In this nitride semiconductor laser chip, the electrode is formed so as to be electrically in contact with the side walls of the ridge portion but out of contact with the semiconductor layer in a side-bottom part of the ridge portion. With this structure, the electric charge resulting from spontaneous polarization and piezoelectric polarization of the nitride semiconductor layer is canceled out, and the operating voltage is reduced. JP-A-2010-34246 also discloses a structure in which the ridge portion is given a ridge width larger than 1.5 μm.
With the structures disclosed in JP-A-2010-34246 mentioned above, however, it is certainly possible to reduce the operating voltage, but, inconveniently, it is difficult to suppress higher-order modes. In particular, in cases where the ridge width is increased, higher-order modes are likely to occur, and this inconveniently tends to result in degraded device characteristics and lower reliability.